This invention relates generally to Doppler acoustic echo sounder (or acoustic radar) systems (also called Doppler sodar or Doppler acdar or Doppler echosonde systems), and more particularly concerns the employment of bistatic transmitting and receiving antennas in such manner as to significantly improve wind measurement performance of the system, and to simplify equipment deployment. The term "acoustic radar" as used herein refers to an acoustic echo sounder.
A Doppler bistatic acoustic wind monitoring system of the pulse type transmits an acoustic pulse upward along a first beam A (defined by the transmit antenna pattern), and then listens for and analyzes echoes returned from atomspheric scatterers. The listening is accomplished along a beam B defined by the receive antenna pattern. A and B beams must be coincident, at least at the scatter range being investigated; in fact, in a monostatic (MS) system, the beams are everywhere coincident because the same antenna is used first as the transmitter and then as the receiver. A bistatic (BS) system is defined as one in which A and B beams are not everywhere coincident.
The Doppler frequency shift of the returned signal is found to be proportional to the component of the velocity of the scatterers along a line bisecting the angle made by the lines extending from the scatterers to the receive antenna and to the transmit antenna. The scatterers are small scale temperature and velocity anomalies that move generally with the local wind. The location and size of the region of the scatterers are determined by the beam geometry (beam width at scatter zone, etc.), by the time interval between transmission of the acoustic pulse and receipt of the echoes, by the pulse length, and by the time constraints in the spectrum analyzer device. Thus, for one set of antennas and beams A and B, it is possible to obtain one velocity component for the local region; for a second antenna pair with different orientation a second velocity component can be found; and a third antenna system will yield a third velocity component. If three components can be found for the same height region, and if the atmosphere can be expected to be horizontally homogeneous, then even if the sensed regions are somewhat displaced horizontally, a representative wind vector V can be derived from the three components (or from two components if we assume the wind is horizontal) using geometrical relationships.
Temperature variations within the sensed region cause back scatter of sound generally at all angles toward the transmitted beam (although not at right angles to the beam); velocity variations do likewise, except for angles around 180.degree. (backscatter direction). Thus, if there is turbulence but no temperature microstructure at the scatter region, a monostatic antenna system will not receive echoes, but a bistatic antenna system will. On the other hand, conventional bistatic systems require excessive spacing of the transmit and receive antennas for surveys of higher scatter regions; also, one of the antennas must be of sufficiently wide beam or have several beams in a fan orientation so as to permit surveys of scatter regions throughout a range of heights.
There are various complex relations between the scattering sensitivity, beam geometry, range, and ambient noise which enter the evaluation of what beam systems will give the best signal/noise ratio, and hence the best range, in a particular meteorological situation--and, for the deviation of a wind vector, there are further considerations of the accuracy consequences pertaining to the geometric relationships between the observed components and the total wind. See in this regard, "Review of Geophysics and Space Physics" Volume 16, No. 1, February 1978, and "Journal of Geophysical Research", Volume 79, No. 36, Dec. 20, 1974, pages 5585-5591. These and other considerations demonostrate the need for simplification of the equipment needed to achieve high performance.